Gaoming Feng

Multilevel Data Storage Characteristics of Phase Change Memory Cell with Doublelayer Chalcogenide Films (Ge2Sb2Te5 and Sb2Te3)

Phase change memory (PCM) cells with monolayer chalcogenide film (Ge2Sb2Te5 or Sb2Te3) and doublelayer chalcogenide films (Ge2Sb2Te5 and Sb2Te3) were successfully fabricated. The PCM cell with doublelayer structure like W/Ge2Sb2Te5(30 nm)/Sb2Te3(60 nm)/TiN/Al shows superior performances to monolayer ones, which is mostly referred to the reduction of set voltage value and reset voltage value, and the ability of multilevel data storage. Theoretical simulations of the temperature distribution of the PCM cell with the doublelayer structure were also investigated.

High speed chalcogenide random access memory based on Si2Sb2Te5

A high speed chalcogenide random access memory based on Si2Sb2Te5, which is outstanding in data retention and electrical performance, has been fabricated by 0.18-µm complementary metal oxide semiconductor technology. set and reset times are 31 and 10 ns when the corresponding voltage pulses are 2 and 3.5 V, respectively. Si2Sb2Te5 material possesses better data retention and lower threshold current comparing to Ge2Sb2Te5 does. Endurance up to 106 cycles with a resistance ratio of 100 has been achieved.

Reactive ion etching of Ge2Sb2Te5 in CHF3/O2 plasma for nonvolatile phase-change memory device

The etching characteristics of Ge 2 Sb 2 Te 5 (GST) films were studied with a CHF 3 /O 2 gas mixture using a reactive ion etching system. The variations of etch rates and etch profiles caused by changes in the gas-mixing ratio were investigated under constant pressure and applying power. Then the etching parameters were optimized. The etch rate is up to 83 nm/min, and the selectivity of GST to SiO 2 is as high as 3 times. The smooth surface was achieved using optimized etching parameters of oxygen concentration of 4% in the CHF 3 /O 2 gas mixture, pressure of 30 mTorr, and power of 150 W.

RESET Distribution Improvement of Phase Change Memory: The Impact of Pre-Programming

Some nonvolatile phase change memory (PCM) cells with 80-nm heating electrodes are found very difficult to RESET at 3 mA, which directly affects the RESET distribution of the PCM. The large crystal grains with hexagonal structure in the active phase change area, discovered by transmission electron microscope, are the major reason. One preprogramming testing method is introduced, and the resistance distributions of the PCM cells before and after preprogramming are presented. Results show that the large hexagonal crystal grains have been eliminated, thus the resistance distributions have been greatly improved after preprogramming.

Understanding the early cycling evolution behaviors for phase change memory application

The RESET current of T-shaped phase change memory cells with 35 nm heating electrodes has been studied to understand the behavior of early cycling evolution. Results show that the RESET current has been significantly reduced after the early cycling evolution (1st RESET) operation. Compared the transmission electron microscope images, it is found that the hexagonal Ge2Sb2Te5 (GST) crystal grains are changed into the grains with face centered cubic structure after the early cycling evolution operation, which is taken as the major reason for the reduced RESET current, confirmed by a two-dimensional finite analysis and ab initio calculations.

Fabrication of the Si 2 Sb 2 Te 5 phase change cell structure for PCRAM by using UV nanoimprint lithography

Phase-change random access memory (PCRAM) is emerging as one of the most promising non-volatile memories for the next generation media due to its fast write/read speed, wide dynamic range, high degree of cycling endurance, excellent data retention, simple structure, low operating voltage, good compatibility with CMOS technologies, and easy applicability. The PCRAM utilizes a reversible phase change phenomena between crystalline and amorphous states of chalcogenide materials by electrical resistive joule heating. A resistance of the crystalline phase (set) is much lower than that of the amorphous phase (reset).Being able to pattern and etch phase change memory in nanometer scale is essential for low power consuming operation of PCRAM device. In particular, high-density electronic memory, uniform, consistent and smooth sidewall storage unit structure for the electrical properties of memory is essential.UV nanoimprinting lithography (UV-NIL) is a new emerging lithographic technique in which patterns as small as sub-100 nm can be easily replicated onto a resin layer from surface protrusions of a stamp with a potential for high throughput at low cost, and a promising as one of the next generation lithography. This study uses the UV-NIL for patterning the PCRAM device. Si wafers coated with SiO2 were used as. substrates. Titanium bottom electrode, TiN contact layer, and Si2Sb2Te5 (SST) were deposited by sputtering method. The thicknesses of Titanium, TiN and SST layer is about 100, 40, and 200 nm, respectively. Patterns of UV imprinting resin were formed using UV-NIL on the surface of SST films, and that were etched using SF6/O2 plasma in a RoTH&RAU MS-350 reactive ion etching (RIE) etcher. The experimental results show that by using UV-NIL processing uniformly consistent, high-quality edge smooth sidewall structure for PCRAM devices is obtained. The operation behaviors of the fabricated devices were characterized by using electrical measurement system, SST material possesses lower threshold current with a resistance ratio of 65 has been achieved.

16kb Phase Change Memory Test Chip with 0.18-μm Process

A 16kb phase change memory with standard CMOS 0.18-μm test chip has been demonstrated. The critical integration technology of the phase change material fabrication and the standard CMOS process has been improved. Full integration of PCRAM, including memory cell, array structure, critical circuit module, and physical layout, has been designed and verified. Test results about PCM cell and circuit module has been generated.